1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system.
2. Description of Related Art
There are known some conventional sheet processing apparatuses each of which forms a booklet with a saddle stitching section including a saddle unit to sequentially accumulate sheets of paper that are mountain-folded along middle folding lines so as to straddle the saddle unit, a staple inserting unit to insert a staple from above into the folding lines of the sheets accumulated on the saddle unit and a staple receiving unit which is provided to face the staple inserting unit across the sheets and receives and bends the ends of the staples inserted by the staple inserting unit (for example, see Japanese Patent Application Laid Open Publication No. 2005-41661).
In Japanese Patent Application Laid Open Publication No. 2005-41661, the staple receiving unit is built in the saddle unit and configured to protrude upward from the top of the saddle unit when the staple inserting unit puts a staple through a sheet.
However, such configuration has a problem that the staple receiving unit is moved upward less stably when a larger number of sheets are accumulated and thus the staple is inserted at a less accurate position.
Against the above problem, there has been suggested a configuration in which the staple receiving unit is fixed inside the saddle unit, the top of the staple receiving unit is located at the top of the saddle unit and the staple inserting unit moves so as to make an arc (hereinafter, called are motion) toward the saddle unit and the staple receiving unit (for example, see Japanese Patent Application Laid Open Publication No. H9-216764).
Such staple inserting unit which makes an arc motion is also used for side stitching processing to put staples into unfolded sheets.
However, since the staple inserting unit makes an are motion, as shown in FIG. 13, the above Japanese Patent Application Laid Open Publication No. H9-216764 has a problem that the staple inserting unit D makes the arc motion with the orbit out of the ridge line at the top of the upmost sheet in the sheet bundle depending on the thickness of the sheet bundle accumulated on the saddle unit K, and thus the staple is inserted at a position (stapling position) shifted from the folding line on the upmost sheet, that is, a desired position.
Such problem also possibly occurs in the side stitching processing.